A master plan amendment would substitute detours along existing bikeways and sidewalks and a 3-mile unpaved trail in the Paint Branch area for segments that would have run through park land and closer to the course of the 18-mile road being built between Interstate 370 in Gaithersburg to Interstate 95 in Laurel.

The proposed changes, which have not been measured, could add roughly 50 percent more mileage to the bicycling route, said Charles Kines, a transportation planner and coordinator for the county Planning Department. A roughly 18-mile route could become 27 miles.

And the 3-mile unpaved trail would be usable only by hikers and riders of mountain bikes, horses or other animals.

The state is to build 7 miles of the bikeway in the ICC highway right of way, separated by a grass strip or barrier.

The state has proposed running the rest of the route along existing bike paths and walkways.

What county planners are saying is ‘‘the state’s plan is valid but we can improve on it,” Kines said.

‘‘We’re opposed to the amendment because it drastically weakens the trail,” said Jack Cochrane of Bethesda, chairman of Montgomery Bicycle Advocates, a group that focuses on using bicycles for transportation.

Decisions about the course and construction of the bike route are made by the jurisdiction or agency where each segment would run.

The Department of Parks, which, like the Department of Planning, is governed by the same five county park and planning commissioners, opposes running the bike route through park land they say is vulnerable to environmental degradation from even limited construction.

The only way to get through the Paint Branch area in an environmentally sound way is to build an elevated boardwalk and for that materials alone would cost $1.5 million, Kines said.

‘‘We’re obviously concerned about the environment but we’re talking about a trail next to a six-lane highway — to say a trail damages [that] environment is absurd,” Cochrane said.

Neal Fitzpatrick, executive director of the Audubon Naturalist Society, said he sees the bike path as a "distraction" given his organization's strong concerns over the road project. The society, Environmental Defense and the Sierra Club, sued to stop the road but a judge ruled against them in November. Environmental Defense and Sierra are appealing the decision.

Fitzpatrick said the state needs to re-evaluate whether it can afford to build the road given that revenue from the gas tax and tolls may decline because people may be cutting their driving to fit gas that costs more than $4 per gallon into their budgets.